WHEN HUMANS, CHIMPS WERE KISSIN' COUSINS / Genetic analysis shakes up ideas on when, how the two species diverged

Published 4:00 am, Thursday, May 18, 2006

WHEN HUMANS, CHIMPS WERE KISSIN' COUSINS / Genetic analysis shakes up ideas on when, how the two species diverged

1 / 2

Back to Gallery

The story of human evolution that began when the earliest ancestors of humans and chimpanzees first separated from their common ancestor has taken a new turn with a claim that the epochal event may well have occurred millions of years more recently than the fossil record has long maintained.

On top of that, the split of the two lineages was probably far more complex than anyone had thought -- with some of the earliest members of both lines interbreeding again and again before the two species finally separated permanently, according to scientists analyzing the genes of both modern chimps and modern humans.

Latest news videos

If that controversial claim turns out to be true, the famed fossil skull of the creature nicknamed Toumai -- and dated at roughly 7 million years old -- may have lived long before the final chimp-human split occurred, say researchers at the Broad Institute, a collaborative venture linking the Massachusetts Institute of Technology and Harvard.

Toumai (Sahelanthropus tchadensis), discovered five years ago in the desert of Chad by French anthropologists, has until now been considered the earliest known hominid representative in the human lineages that split from a common ancestor of humans and chimps.

But by analyzing the genes of modern humans and chimps and other primates, the MIT and Harvard researchers say, the true split between the ancestral ape and human lineages could have begun much later -- probably much less than 5.4 million years ago and surely no more than 6.3 million years ago -- than previous estimates based on the fossil record.

The new estimate for the earliest beginnings of human evolution was published online Wednesday in the journal Nature by a group led by David Reich, a geneticist at the Broad Institute, in Cambridge, Mass. The group includes famed geneticist Eric Lander, a leader of the Human Genome Project and the Broad Institute's director.

The scientists analyzed the genes in the X chromosomes of humans, chimps, gorillas, orangutan and macaques -- the chromosomes carried by the male and female lines of those animals. They noted that creatures like Toumai, which displayed both human and chimp features and has been set at between 6.5 million and 7.4 million years old, could well have come from a species whose members were interbreeding before the final separation between the lineages of human and chimp species occurred.

"The study gave unexpected results about how we separated from our closest relatives, the chimpanzees," Reich said in a statement. "We found that the population structure that existed around the time of human-chimpanzee speciation was unlike any modern ape population. Something very unusual happened at the time of speciation."

The unusual happening was that once the two species began separating from their common ancestors, they might well have been repeatedly interbreeding with each other -- at times producing sterile offspring but sometimes producing offspring that remained fertile and, in turn, created a short-lived mixed lineage -- a kind of dead-end tribe combining both pre-human and pre-chimp genes.

It was one of those interbred hybrid groups that ultimately began the human lineage, according to the Broad Institute group.

"It's a fascinating tale of ancient shenanigans," said David Haussler, a leading geneticist at UC Santa Cruz in an e-mail message. "I continue to be amazed at what DNA analysis can reveal."

Edward M. Rubin, director of the Department of Energy's Joint Genome Institute and chief of genetics at the Lawrence Berkeley Laboratory, was equally intrigued.

Rubin, who has been interested in determining the genetic relationship between Neanderthals and modern humans, called work of the Reich team "complicated, but plausible."

"Until now our view of human evolution has largely come from the fossil record which is still incomplete," he said. "This is very provocative because it provides a new window into a time when hominids separated from the common ancestor of chimpanzees. It's the beginning of a new wave of work that will bring us new insights into our view of how species separate."

Rubin noted that the Broad Institute team worked with only 20 million base pairs of genes in their genetic analysis of the X chromosomes of humans and the other primates, while some 3 billion base pairs are available in the human genome alone. Even firmer conclusions will come, he said, when the entire genome of the gorilla has been determined, as this will provide evidence for the far more ancient time when the gorilla and chimp lineages separated.

At the Broad Institute, Nick Patterson, a statistical geneticist, noted that "if the (fossil) dating is correct, the Toumai fossil would precede the human-chimp split. The fact that it has human-chimp features suggests that human-chimp speciation may have occurred over a long period of time with episodes of hybridization between the emerging species."

Latest from the SFGATE homepage:

Click below for the top news from around the Bay Area and beyond. Sign up for our newsletters to be the first to learn about breaking news and more. Go to 'Sign In' and 'Manage Profile' at the top of the page.